Actuators are extensively used in mechanical, electronic and optical systems like positioning lenses in telescopes, focusing in microscopes or vibration suppression in dedicated equipment. Traditional actuation by pneumatic, hydraulic or electromagnetic means cannot fulfill modern machinery requirements in terms of response time, frequency, resolution and size. Thus, several classes of actuators using piezoelectric, magnetostrictive and shape memory alloy smart materials have been developed for these applications.
The construction of these actuators is usually based on bulky, fragile and expensive smart material elements like piezoelectric multilayer stacks and complex and expensive mechanical amplifiers, which make these actuators vulnerable to damage and very complex and expensive. Among the piezoelectric actuators, unimorph/multi-morph type devices as illustrated in FIG. 1(a) are the most competitive in terms of size, weight, cost, reliability and manufacturing. These kinds of devices 100 comprise alternating layers of piezoelectric materials 102 and substrate materials 104 bonded together or grown by sputtering/chemical processes. The piezoelectric materials 102 are mostly piezoelectric ceramics and the substrate materials 104 can be made of metals, polymers, ceramics or their composites. When an electrical signal is applied across the piezoelectric materials, the piezoelectric materials change their shapes, causing the multiple layers of materials to deform. Additionally, these types of devices can also generate electrical signals when loads are applied on them. This two-way capability makes these unimorph/multi-morph devices suitable for use as actuators, sensors and/or sensoriactuators. Unimorph/multi-morph devices are well known to be capable of providing large displacement output. However, their blocked forces are relatively small due to their small flexural strength, and their applications are generally limited to those requiring smaller loading.
An example of a type of piezoelectric actuator is described in U.S. Pat. No. 4,760,570 entitled “Piezo-Electric Device”. The patent describes a device with piezoelectric ceramic plates sandwiching an intermediate plate. The device is used as a vibration source for delivering vibrational motions. However, it has a limited loading capacity, as explained above. Another example of a prior art piezoelectric actuator is described in U.S. Pat. No. 4,952,835 entitled, “Double Saggital Push Stroke Amplifier”. This patent describes a mechanism that is applicable in high performance piezoelectric actuators. The geometric profile of this device resulting from its amplification scheme can provide a relatively high displacement output and higher forces when a piezoelectric ceramic multilayer stack is used. Nevertheless, it is vulnerable to damage and is expensive to produce.
To achieve higher loading, a descendent of the unimorph devices, such as a descendent device 106 shown in FIG. 1(b) utilizes a dome-shaped architecture wherein a piezoelectric material 102 is bonded to a curved substrate 108 formed at elevated temperatures. The curvature is inherently a compact mechanical amplifier which provides the device with comparably higher displacement and force output than unimorph/multimorph devices. Such architecture is described, for example, in U.S. Pat. No. 6,734,603 entitled “Thin Layer Composite Unimorph Ferroelectric Driver and Sensor”.
Although this descendent device 106 possesses better performance, it suffers from installation problems due to its inherent curvature which makes the device performance sensitive to mounting. Reliability of the device under operation is also worsened due to the concentrated loading on the curved surface of the device, which induces stress concentration and rubbing on the brittle piezoelectric ceramic layers. Thus, it requires extra peripheral components specially designed for the descendent device if the device has to be coupled to an external object.